1. Field of the Invention
The invention relates to the field of etchants for silicon.
2. Prior Art
Prior art etching solutions for silicon are of essentially two types. The first type are basic solutions such as sodium hydroxide or potassium hydroxide. These basic solutions rapidly attack photoresist material, thus preventing use of etching masks formed of photoresists. However, these solutions attack silicon oxide much more slowly than they attack silicon. Consequently, it has become standard practice in the industry to utilize a silicon oxide etching mask to define and protect the area of the silicon which is not to be etched when these basic etching solutions are utilized. Unfortunately, these basic solutions when utilized with a silicon oxide mask yield an undesirable undercut within the "protected" silicon in the vicinity of the oxide mask. This undercut can be detrimental to the quality of integrated circuits produced using this process because the undercut tends to result in voids in oxide or metal deposited over the etched silicon.
A further disadvantage of hydroxide etching solutions is the fact that these solutions etch N.sup.+ monocrystalline silicon approximately 1.5 times faster than they etch P.sup.+ monocrystalline silicon. As a result of this difference in etch rate, a single immersion in the etching bath will not etch N.sup.+ and P.sup.+ silicon to the same depth. Further, these etching systems are anisotropic and consequently etch monocrystalline silicon faster parallel to some crystallographic axes than they etch monocrystalline silicon parallel to other crystallographic axes.
The second type of prior art silicon etching solutions are acid solutions containing nitric acid, hydrofluoric acid and acetic acid. These etchants rapidly attack photoresist materials and result in an etch having a profile which includes an undesirable undercut within the protected silicon. Since these etchants attack photoresists, it is necessary to utilize an etching mask such as thermally grown silicon oxide or deposited silox or some other masking material which is more resistant to the etchant than silicon is.
The requirement that silicon oxide masks be utilized when etching silicon increases process time and costs because the silicon wafer to be etched must first be introduced into a furnace for the purpose of growing or depositing the silicon oxide masking material prior to its definition by a photoresist and etching technique.